Short Answer:

Polymers behave differently under mechanical loads compared to metals. When force is applied, some polymers stretch and return to their original shape (elastic behavior), while others may permanently deform or break (plastic or brittle behavior). Their response depends on the type of polymer, temperature, load type, and how fast the load is applied.

Under low loads, most polymers show elastic behavior, but under high or repeated loads, they may show creep, stress relaxation, or fracture. Some polymers are soft and flexible, while others are strong and tough, making them useful for a wide range of applications like packaging, insulation, automotive parts, and medical devices.

Detailed Explanation:

Polymer behavior under mechanical loads

When mechanical force or stress is applied to a polymer, it reacts in various ways depending on its molecular structure, temperature, and type of loading. These behaviors are important in designing products that must handle stress without failing, such as plastic parts, rubber components, and medical devices.

Polymers can be thermoplastics, thermosetting plastics, or elastomers, and each group behaves differently under load.

1. Elastic and plastic deformation

• Elastic behavior:
  When a small load is applied, polymers stretch slightly and return to their original shape after the load is removed.
  Example: Rubber bands, soft plastics.
• Plastic behavior:
  At higher loads, some polymers undergo permanent deformation and do not return to their original shape.
  Example: Stretching a plastic bottle until it doesn’t go back.
• Thermoplastics often show both elastic and plastic deformation depending on the load.

2. Creep

• Creep is the slow and continuous deformation of a polymer when a constant load is applied over time.
• Common in applications like pipe fittings, gaskets, or cables that bear constant stress.
• Polymers show more creep than metals, especially at high temperatures.

3. Stress relaxation

• When a polymer is stretched and held at a fixed position, the internal stress decreases slowly with time.
• This is known as stress relaxation and is important in seals, joints, and fasteners where stress must remain low.

4. Viscoelastic behavior

• Polymers exhibit both viscous and elastic characteristics, meaning they flow like a liquid and stretch like a rubber at the same time.
• Their response depends on how quickly the load is applied:
  • Fast loading → more brittle response
  • Slow loading → more flexible or rubbery behavior

5. Effect of temperature

• Polymers are highly sensitive to temperature:
  • At low temperatures → they become hard and brittle.
  • At room temperature → they are usually flexible and elastic.
  • At high temperatures → they soften and deform easily.
• The glass transition temperature (Tg) is the point where a polymer changes from hard to rubbery.
  • Below Tg: brittle
  • Above Tg: soft and elastic

6. Fracture behavior

• Some polymers can absorb energy and deform before breaking (tough behavior), while others break suddenly (brittle).
• Tough polymers: Polycarbonate, ABS
• Brittle polymers: Polystyrene, some thermosets

7. Effect of additives and reinforcement

• Adding fibers or fillers improves polymer strength under load.
• Fiber-reinforced polymers are used in automotive parts, aerospace, and construction due to better load-carrying ability.

Real-life applications

• Rubber tires: Handle repeated loading and deformation without failure.
• Plastic containers: Designed to resist crushing and stretching.
• Medical implants: Must maintain shape under body forces.
• Insulation materials: Should resist long-term creep and sagging.

Conclusion

Polymers show a variety of behaviors under mechanical loads including elasticity, plasticity, creep, stress relaxation, and fracture. Their response depends on the type of polymer, temperature, and the way the load is applied. Understanding these behaviors is important to ensure safety, performance, and durability in mechanical engineering applications. With proper selection and design, polymers can be used effectively in both soft and load-bearing components across industries.